Interface circuit and operating method thereof

ABSTRACT

An interface circuit has a load, a driving circuit, and a bias circuit and outputs an output signal in accordance with an input signal between both ends of a load resistor. The driving circuit has a first MOS transistor which supplies a first constant current and a bridge circuit which supplies the first constant current to the load switchingly. The bias circuit has a fixed resistor, a second MOS transistor which is connected with the fixed resistor and which is operable with the first transistor under a Miller effect; and a differential amplifier whose non-inverting input terminal receives a predetermined voltage whose inverting input terminal receives a voltage of a connection node between the second transistor and the fixed resistor, and an output terminal which applies an output voltage at the control terminals of the first and second transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an interface circuit which transmits a digital signal at a high speed.

2. Description of the Related Art

Recently, it is increasingly demanded that interface circuits, employed in I/O sections of semiconductor integrated circuits, be operated at a high speed with less noise. Examples of high-speed interface circuits are a small-amplitude differential output circuit, typically known as an LVDS (Low Voltage Differential Signalling), and a small-amplitude output circuit, typically known as a GTL (Gunning Transceiver Logic) or an HSTL (High Speed Transceiver Logic).

FIG. 1 is a circuitry diagram showing the structure of an example of an LVDS type interface circuit. The first example of the interface circuit comprises: a drive circuit 7 which differentially outputs an output signal in accordance with an input signal V_(IN) to a terminating resistor R_(L) connected between two output terminals D_(O) and X_(DO); and a bias circuit 6 which controls an output current I of the drive circuit 7.

The drive circuit 7 comprises: a buffer 71 which performs non-inverting output of an input signal V_(IN); an inverter 72 which performs inverting output of an input signal V_(IN); a P-channel MOSFET (hereinafter referred to as an P-MOSFET) P₁₁ and an N-channel MOSFET (hereinafter referred to as an N-MOSFET) N₁₁ which are driven by the buffer, a P-MOSFET P₁₂ and an N-MOSFET N₁₂ which are driven by the inverter, and a P-MOSFET P_(C11) which serves as a constant current source for making a predetermined output current flow to the terminating resistor R_(L) connected between the two output terminals D_(O) and X_(DO).

The bias circuit 6 comprises: a fixed resistor R_(P11); and a P-MOSFET P_(X11) which constantly controls a current I_(RP) to flow to the fixed resistor R_(P11).

In such a structure, when the input signal V_(IN) is at a low level, the P-MOSFET P₁₁ is ON, and the N-MOSFET N₁₁ is OFF, the P-MOSFET P₁₂ is OFF, and the N-MOSFET N₁₂ is ON. Thus, as described iwth arrow D in FIG.> 1, the output current I flows through a path along the P-MOSFET P_(C11), the P-MOSFET P₁₁ and the N-MOSFET N₁₂. At his time, a low level voltage (V_(CL)) is out put to the output terminal D_(O), whereas a high level voltage (V_(OH)) is out put to the output terminal X_(DO).

In the first example of the interface circuit, the P-MOSFET P_(X11) and the P-MOSFET P_(C11) operate in their saturation range, and the dimensions of the respective transistors are designed such that constants of the transistors are set at a predetermined ratio. In this structure, the P-MOSFET P_(X11) and the P-MOSFET P_(C11) operate under the Miller effect, thus a current I_(RP) flowing through the P-MOSFET P_(X11) and a current I flowing through the P-MOSFET P_(C11) are in proportion to each other

Accordingly, when having the structure of the bias circuit 6 as shown in FIG. 1, any variation in the current I_(RP) which may occur as a result of a variation in the source voltage V_(DD) or any difference (deviation) occurring in transistors in the manufacturing processes can be reduced. In addition, a variation in the output current I of the drive circuit 7 which is in proportion to the current I_(RP) of the bias circuit 6 can be reduced.

FIG. 2 is a circuitry diagram showing the structure of the second example of an interface circuit which is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H3-283713. The second example of the interface circuit shown in FIG. 2 comprises: a drive circuit 9 comprising a P-MOSFET P₁₅ and an N-MOSFET N₁₃ which are connected in series between a power source V_(DD) and a ground potential; an NAND circuit 83 which supplies the P-MOSFET P₁₃ in the drive circuit 9 with a gate voltage V_(PO); a NOR circuit 84 which supplies the N-MOSFET N₁₃ with a gate voltage V_(NG); and a sense amplifier 81 and a sense amplifier 82 which control a voltage of an output terminal D_(O) in the drive circuit 9 to be in a predetermined value. The output terminal D_(O) is connected to a reference voltage V_(TT) along a transmission path via a terminating resistor R_(L).

In such a structure, a control voltage V_(OH) for controlling a voltage of the output terminal D_(O) in the drive circuit 9 into a high level is input to a non-inverting input terminal of the sense amplifier 81 serving as a differential amplifier. A control voltage V_(OL) for controlling a voltage of the output terminal D_(O) in the drive circuit 9 into a low level is input to a non-inverting input terminal of the sense amplifier 82 serving as a differential amplifier.

The voltage of the output terminal D_(O) is fed back to inverting input terminals of the respective sense amplifiers 81 and 82. Thus, the sense amplifier 81 controls the voltage of the output terminal D_(O) to a voltage level of V_(OH), whereas the sense amplifier 82 controls the voltage of the output terminal D_(O) to a voltage level of V_(OL).

Either one of the NAND circuit 83 and the NOR circuit 84 supplies the MOSFET included in the drive circuit 9 with a gate voltage, in accordance with the conditions of the input signal V_(IN). The output voltages of the respective NAND circuit 83 and NOR circuit 84 are so controlled that their voltage values are in proportion to the value of the output voltages of the respective sense amps 81 and 82.

Therefore, when the input signal V_(IN) is at a high level, the voltage of the output terminal D_(O) is controlled to be at a voltage level of V_(OH) by the sense amp 81, resulting in making the current I_(H) flow to the terminating resistor R_(L). On the contrary, when the input signal V_(IN) is at a low level, the voltage of the output terminal D_(O) is controlled at a voltage level of V_(OL), resulting in making the current I_(L) flow to the terminating resistor R_(L).

Accordingly, having controlled the output current I_(L) or I_(H) to flow to the terminating resistor R_(L) in accordance with the conditions of the input signal V_(IN), the voltage of the output terminal D_(O) varies.

In the first example of an interface circuit shown in FIG. 1, a variation of the current I_(RP), resulting from a source voltage variation or any difference occurring in transistors in the manufacturing processes, can not sufficiently be reduced. This entails problems that a current variation is large in the output current I and an amplification variations is also large in the output voltage.

Variations in the current I_(RP), which variations occur in the bias circuit as a result of a variation in a source voltage or any difference occurring in transistors in the manufacturing processes, will now be explained with reference to FIGS. 3 and 4.

FIG. 3 is a circuitry diagram showing the structural example of a bias circuit employed in an interface circuit. FIG. 4 is a graph showing characteristics of output currents and output voltages with reference voltage of a gate voltage to be applied to the bias circuit illustrated in FIG. 3.

In the bias circuit shown in FIG. 3, a P-MOSFET P_(X12) and a fixed resistor R_(P12) are connected in series between a power source V_(DD) and a ground potential. In such a structure, when the power source V_(DD) is set at 3.6 V or 2.7 V, the relationship between the current I_(RP) and the output voltage V_(RP) with reference to the gate voltage V_(O) is as shown in FIG. 4.

In the first example, as shown in FIG. 1, of the bias circuit, the gate voltage and the drain voltage of the P-MOSFET P_(X11) are the same (V_(GP)). Accordingly, based on the characteristics shown in FIG. 4, the current I_(RP) is −1.9 mA when the source voltage VDD is 2.7 V, whereas the current I_(RP) is −3.3 mA when the source voltage V_(DD) is 3.6 V.

Accordingly, in the bias circuit having the structure shown in FIG. 1, when the source voltage V_(DD) varies from 2.7 V to 3.6 V, the output current I of the interface circuit which is in proportion to the current I_(RP) varies as well.

In the second example of the interface circuit shown in FIG. 2, when the input signal V_(IN) to be output, there is a delay before the voltage of the output terminal D_(O) varies in response to the variation in the output voltage of the NAND circuit 83 or NOR circuit 84. Further, there is a delay before an output voltage of the NAND circuit or NOR circuit varies when the sense amp 81 or 82 responses to the variation in the voltage of the output terminal D_(O). Accordingly, when the voltage of the output terminal D_(O) is switched from V_(OH) to V_(OL) or from V_(OL) to V_(OH), a drawback is that a noise occurs.

FIG. 5 is a waveform diagram showing operations of the interface circuit shown in FIG. 2. Illustrated in FIG. 5 are the gate voltage of the output terminal D_(O) and the gate voltage of the P-MOSFET, when the voltage of the output terminal D_(O) switches from V_(OL) to V_(OH). As shown in FIG. 5, in response to switching of the voltage of the output terminal D_(O) from V_(OL) to V_(OH), the sense amp 81 makes the output voltage V_(PG) of the NAND circuit 83 vary with a delay of a delay period T₁, along with an increase in the voltage of the output terminal D_(O). Hence, the voltage of the output terminal D_(O) continuously increases during the period of T₁+T₂, even if it exceeds the control voltage V_(OH), resulting in generating a spike noise. Similarly, when the voltage of the output terminal D_(O) switches from V_(OH) to V_(O), a noise is generated.

In the second example of the interface circuit, two sense amps are necessary, causing a large size of the circuit to be manufactured.

As a technique having relevance to the present invention, for example, Unexamined Japanese Patent Application KOKAI Publication No. S61-244120 discloses a logical signal detecting output circuit which converts an output signal of a differential amplifier into an ECL level signal. Unexamined Japanese Patent Application KOKAI Publication No. H2-27807 discloses a technique for a differential amplifier, wherein the Miller capacitance is set small and the frequency characteristic of the amplifier is set at a broad band. Further, Unexamined Japanese Patent Application KOKAI Publication No. H5-327472 discloses an output circuit for obtaining a large driving current and voltage amplification. Unexamined Japanese Patent Application KOKAI Publication No. H6-326591 discloses an output circuit which can perform small-amplification operations to operate at a high speed. Furthermore, Unexamined Japanese Patent Application KOKAI Publication No. H9-8637 discloses an output circuit for outputting an accurate output signal without any high accuracy resistance. However, the techniques discloses in these publications do not solve the above mentioned problems of the first and second examples of the interface circuits.

SUMMARY OF THE INVENTION

The first object of the present invention is to provide an interface circuit, wherein a variation in an output current which variation occurs as a result of a variation in a source voltage or any difference occurring in transistors in the manufacturing processes, is controlled.

The second object of the present invention is to provide an interface circuit, wherein a variation in amplification of an output current, which variation occurs as a result of a variation in a source voltage or any difference occurring in transistors in the manufacturing processes, is controlled.

The third object of the present invention is to provide an interface circuit having a simple structure and being operable at speed.

The fourth object of the present invention is to provide an interface circuit wherein any noise is prevented from occurring and which is operable at speed.

In order to achieve the above-described objects, according to the first aspect of the present invention, there is provided an interface circuit which outputs an output signal in accordance with an input signal to at least one end of a load, the interface circuit comprising:

a load;

a driving circuit having a first resistor which supplies a first constant current to the load and a switching circuit which supplies the first constant current to the load; and

a bias circuit having a fixed resistor, a second transistor which is connected with the fixed resistor and which is operable with the first transistor under a Miller effect, and a control circuit which controls a voltage applied to control terminals of the first and second transistor and an output voltage of the second transistor to be at predetermined voltage ratio.

In the interface circuit.

each of the first and second transistor may include a current path and a control terminal;

the current path of the first transistor may be connected to the switching circuit;

the current path of the second transistor may be connected to the fixed resistor; and

the interface circuit may comprise a differential amplifier whose non-inverting input terminal receives a predetermined voltage, and whose inverting input terminal receives a voltage of a connection node between the second transistor and the fixed resistor, and a output terminal which applies an output voltage at the control terminals of the first and second transistor.

In the interface circuit,

the switching circuit may switch a flow direction of a current flowing through the load from one end to the other end and from the other end to the one end.

The interface circuit may further comprise:

a second driving circuit having a third transistor which supplies a second constant current to the load and a second switching circuit which supplies the second constant current to the load;

a second bias circuit having a second fixed resistor, a fourth transistor which is connected with the second fixed resistor and which is operable when the third transistor under a Miller effect; and a second control circuit which controls a voltage applied to the control terminals of the third and fourth transistors and an output voltage of the fourth transistor to be at a predetermined voltage ratio.

In the interface circuit:

each of the third and fourth transistors may include a current path and a control terminal;

the current path of the third transistor may be connected to the second switching circuit;

the current path of the fourth transistor may be connected to the second fixed resistor;

the interface circuit further may comprise a second differential amplifier whose non-inverting input terminal receives a second predetermined voltage, and whose inverting input terminal receives a voltage of a connection node between the fourth transistor and the second fixed resistor, and an output terminal which applies an output voltage at the control terminals of the third and fourth transistors.

In the interface circuit,

the first and second transistors may respectively comprise P-channel MOS FETs or N-channel MOS FETS.

In the interface circuit:

each of the first and second transistors may respectively comprise P-channel MOS FETs or N-channel MOS FETS: and

the third and fourth transistors may respectively comprise N-channel MOS FETs or P-channel MOS FETs.

According to the second aspect of the present invention, there is provided an operating method of an interface circuit which outputs an output signal in accordance with an input signal to at least one end of a load, the operating method comprising:

controlling and applying a common voltage to gates of a first transistor and a second transistor whose current path is connected to a resistor so that a voltage at a connection point of the resistor and the second transistor is a predetermined value, thereby to make the first and second transistors operate under a Miller effect; and

supplying, in response to an input signal, a current which flows through the first transistor to a load, thereby outputting an output signal between both ends of the load or at one end of the load.

The operating method of an interface circuit may further comprise:

controlling and applying a second common voltage to gates of a third transistor and a fourth transistor whose current path is connected to a second resistor so that a voltage at an connection point of the second resistor and the fourth transistor is second predetermined value, thereby to make the third and fourth transistor operate under a Miller effect; and

supplying, in response to an input signal, a current which switchingly flows through the first transistor and the third transistor to the load.

BRIEF DESCRIPTION OF THE DRAWINGS

These objects and other objects and advantages of the present invention will become more apparent upon reading of the following detailed description and the accompanying drawings in which:

FIG. 1 is a circuitry diagram showing the structure of the first example of an interface circuit;

FIG. 2 is a circuitry diagram showing the structure of the second example of an interface circuit;

FIG. 3 is a circuitry diagram showing an example of the structure of a bias circuit;

FIG. 4 is a graph showing the characteristics of an output current and an output voltage with a reference voltage of a gate voltage to be applied onto the bias circuit shown in FIG. 3;

FIG. 5 is a waveform diagram showing operations of the interface circuit shown in FIG. 2;

FIG. 6 is a circuitry diagram showing the structure of an interface circuit according to the first embodiment of the present invention;

FIG. 7 is an input-output waveform diagram showing operations of the interface circuit shown in FIG. 6;

FIG. 8 is a circuitry diagram showing another structure of the interface circuit according to the first embodiment of the present invention;

FIG. 9 is a circuitry diagram showing the structure of an interface circuit according to the second embodiment of the present invention; and

FIG. 10 is an input-output waveform diagram showing operations of the interface circuit shown in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explanations will now specifically be made to embodiments of the present invention with reference to the drawings.

First Embodiment

FIG. 6 is a circuitry diagram showing the structure of the interface circuit according to the first embodiment of the invention. The interface circuit, illustrated in FIG. 6, according to the first embodiment comprises: a drive circuit 1 which differentially outputs an output signal in accordance with an input signal V_(IN) to a terminating resistor R_(L) which is connected between two output terminals D_(O) and X_(DO); and a bias circuit 2 which constantly controls an output current I applied to the drive circuit 1.

The drive circuit 1 comprises: a buffer 11 which performs non-inverting output of an input signal V_(IN); an inverter 12 which performs inverting output of an input signal V_(IN); an P-MOSFET P₁ and an N-MOSFET N₁ which are activated by the buffer 11; a P-MOSFET P₂ and an N-MOSFET N₂ which are activated by the inverter 12; and a P-MOSFET P_(CL) which serves as a constant current source for making a given output current I flow to and through the terminating resister R_(L) connected between the two output terminals D_(O) and X_(DO).

The bias circuit 2 comprises: a fixed resistor R_(P1) which is connected between a power source V_(DD) and a ground potential in series; a P-MOSFET P_(X1) which is connected between the power source V_(DD) and the fixed resistor R_(P1) for controlling a current I_(RP) constantly flowing to and through the fixed resistor R_(P1); and a sense amp 21 for controlling a current I_(RP) constantly flowing to and through the fixed resistor R_(P1).

Applied to an inverting input of the sense amp 2 is a control voltage V_(P) for making a desired output current I flow to and through the terminating resister R_(L). An output voltage V_(RP) output from the P-MOSFET P_(X1) is fed back to a non-inverting input of the sense amp 21. An output terminal of the sense amp 21 is connected to the gate of the P-MOSFET P_(X1).

In the structure illustrated in FIG. 6, the sense amp 21, serving as a differential amplifier, operates in such a way that an output voltage V_(RP) of the P-MOSFET P_(X1) and a control voltage V_(P) coincide with each other. The P-MOSFET P_(X1) operates in a saturation range. Thus, the current I_(RP) flowing through the fixed resistor R_(P1) is controlled to regularly be constant, without being effected by any variation in the source voltage V_(DD) or any difference occurring in transistors in the manufacturing processes.

In FIG. 6, the sense amp 21 has the structure, in which it operates in such a way that the output voltage V_(RP) of the P-MOSFET P_(X1) and the control voltage V_(P) coincide with each other. However, the structure of the sense amp 21 is not limited to this, instead, the structure of the sense amp 21 may arbitrarily be determined as long as voltage values of the output voltage V_(RP) of the P-MOSFET P_(X1) and the control voltage V_(P) are set at a predetermined ratio.

FIG. 7 is an input-output waveform diagram showing operations of the interface circuit shown in FIG. 6. As illustrated in FIG. 7, when an input signal V_(IN) is at a high level, any output signal from the buffer 11 is at a high level, while any output signal of the inverter 12 is at a low level. In this structure, the P-MOSFET P₁ is OFF, the N-MOSFET N₁ is ON, the P-MOSFET P₂ is ON and the N-MOSFET N₂ is OFF. Hence, the output current I flows, as described with an arrow A in FIG. 6, through a path along the P-MOSFET P_(Cl), the P-MOSFET P₂ and the N-MOSFET N₁. At this time, a high level voltage (V_(OH)) is output to the output terminal D_(O), whereas a low level voltage (V_(OL)) is output to the output terminal X_(DO).

When the input signal V_(IN) is at a low level, any output signal of the buffer 11 is at a low level, while any output signal of the inverter 12 is at a high level. In this structure, the P-MOSFET P₁ is ON, the N-MOSFET N₁ is OFF, the P-MOSFET P₂ is OFF, the N-MOSFET N₂ is ON. Thus, the output current I flows through a path along the P-MOSFET P_(C1), the P-MOSFET P₁ and the N-MOSFET N₂. At this time, a low level voltage (V_(OL)) is output to the output terminal D_(O), whereas a high level voltage (V_(OH)) is output to the output terminal X_(DO).

In the interface circuit, illustrated in FIG. 6, according to the first embodiment, likewise in the first example shown in FIG. 1, the P-MOSFET P_(X1) of the bias circuit 2 and the P-MOSFET P_(CL) of the drive circuit 1 are operable in their saturation range, and the dimensions of the respective transistors are designed in such a way that their constants are set at a predetermined ratio.

Applied to the gate of the P-MOSFET P_(C1) of the drive circuit 1 is a voltage V_(OP) having the same voltage value as that to be applied to the gate of the P-MOSFET P_(X1) of the bias circuit 2. The P-MOSFET P_(X1) and the P-MOSFET P_(C1) are operable under a Miller effect, thus current values of the current I_(RP) flowing through the P-MOSFET P_(X1) and of the current I flowing through the P-MOSFET P_(C1) are in proportion to each other.

When on-state resistance of the P-MOSFET P_(X11) increases owing to a variation in a source voltage V_(DD) or any difference (derivation) occurring in transistors in the manufacturing processes, a gate voltage V_(OP) of the P-MOSFET P_(X11) increases, resulting in decreasing the on-state resistance. On the contrary, when the on-state resistance of the P-MOSFET P_(X11) decreases, the gate voltage V_(OP) of the P-MOSFET P_(X11) decreases, resulting in increasing the on-state resistance.

Accordingly, by the operations of the sense amp 21 which constantly controls the current I_(RP) to flow, a variation in the current I_(RF), as a result of any variation in the source voltage V_(DD) or any difference occurring in the transistors in the manufacturing processes, can be controlled. Similarly, any variation in the output current I, flowing through the drive circuit 1 which current is proportional to the current I_(RP) flowing through the bias circuit 2, can be controlled as well. Since such variation in the output current I can be reduced, the power to be consumed by the circuit can be maintained at an optimum value. Further, any variation in amplitude of the output voltage can be controlled, thus the transmission speed at which signals are to be transmitted can be retained at an optimum value.

In addition, the interface circuit according to the first embodiment can stably be operable, because it does not have the structure wherein the voltages applied to the output terminals D_(O) and X_(DO) included in the drive circuit 1 are fed back to the sense amp 21.

Accordingly, even if there are a plurality of the drive circuits 1 shown in FIG. 6, a gate voltage V_(GP) can be applied to each constant current source P-MOSFET. Therefore, output currents I flowing through the respective P-MOSFETs can be controlled in the same manner.

In FIG. 6, the P-MOSFET P_(C1) as a constant current source, as included in the drive circuit, is employed, however, an N-MOSFET may be adopted instead.

FIG. 8 is a circuitry diagram showing another structure of the interface circuit according to the first embodiment of the present invention. The interface circuit illustrated in FIG. 8 comprises a drive circuit 101 and a bias circuit 102.

The drive circuit 101 comprises: a buffer 111 which performs non-inverting output of an input signal V_(IN); an inverter 112 which performs inverting output of an input signal V_(IN); a P-MOSFET P₁and an N-MOSFET N₁ which are driven by the buffer 111; a P-MOSFET P₂ and an N-MOSFET N₂ which are driven by the inverter 112; and an N-MOSFET N_(C1) serving as a constant current source for making an output current I flow to and through a terminating resister R₁which is connected between two output terminals D_(O) and X_(DO).

The bias circuit 102 comprises: an N-MOSFET N_(X1) and a fixed register R_(NI) which are connected in series between a power source V_(DD) and a ground potential; and a sense amp 22 which makes a constant current I_(RN) flow through the N-MOSFET N_(X1) and the fixed resistor R_(N1).

Applied to an inverting input terminal of the sense amp 22 is a control voltage V_(N) for making an output current 1 flow to and through the terminating resistor R₁. A voltage V_(MN) output from the N-MOSFET N_(X1) is fed back to a non-inverting input terminal of the sense amp 22. An output terminal of the sense amp 22 is connected to the gate of the N-MOSFET N_(X1).

Second Embodiment

FIG. 9 is a circuitry diagram showing the structure of an interface circuit according to the second embodiment of the present invention. The interface circuit, illustrated in FIG. 9, according to the second embodiment has the structure, in which an output terminal D_(O) is connected to a reference voltage V_(TT) at one end of a transmission path via a terminating resistor R_(L). Further, the interface circuit comprises: a drive circuit 5 which makes an output current I_(L) or I_(H) flow to and through the terminating resistor R_(L) and makes a voltage of the output terminal D_(O) vary, in accordance with the conditions of an input signal V_(IN); and bias circuits 3 and 4 which sends gate voltages for making a constant current flow to and through two constant current sources included in the drive circuit 5.

The drive circuit 5 comprises: a P-MOSFET P₃ and an N-MOSFET N₃ which are connected with each other i series; a P-MOSFET P_(C2) which serves as a constant current source for making a predetermined output current I_(H) flow to and through the terminating resistor R_(L) via the P-MOSFET P₃ and the N-MOSFET N₃; N-MOSFET N_(C2) which serves as a constant current source for making a predetermined output current I_(l) flow to and through the terminating resistor R_(L) via the P-MOSFET P₃ and the N-MOSFET N₃; and an inverter 51 which inverts an input signal V_(IN) and applies gate voltages respectively to the P-MOSFET P₃ and the N-MOSFET N₃.

The bias circuit 3 comprises: a P-MOSFET P_(X2) and a fixed resistor R_(P) which are connected in series between a power source V_(DD) and a ground potential; and a sense amp 31 which constantly controls a current I_(RP) to flow to and through the P-MOSFETP_(X2) and the fixed resistor R_(P).

A control voltage V_(P) corresponding to a predetermined output current I_(H) flowing to the terminating resistor R_(L) is applied to an inverting input terminal of the sense amp 31. An output voltage V_(RP) output from the P-MOSFET P_(X2) is applied to a non-inverting input terminal of the sense amp 31. An output terminal of the sense amp 31 is connected to the gate of the P-MOSFET P_(X2). The bias circuit 3 sends a gate voltage to the gate of the P-MOSFET P_(C2) so that a current I_(H) constantly flow through the P-MOSFET P_(C2).

The bias circuit 4 comprises: an N-MOSFET N_(X2) and a fixed resistor R_(N2) which are connected in series between a reference voltage V_(TT) at one end of a transmission path and a ground potential; and a sense amp 41 which constantly controls a current I_(RN) to flow to and through the N-MOSFET N_(X2) and the fixed resistor R_(N2).

A control voltage V_(N) corresponding to a predetermined output current I_(L) flowing to the terminating resistor R_(L) is applied to an inverting input terminal of the sense amp 41. An output voltage V_(RN) output from the N-MOSFET N_(X2) is fed back to a non-inverting input terminal of the sense amp 41. An output terminal of the sense amp 41 is connected to the gate of the N-MOSFET N_(X2). The bias circuit 4 sends a gate voltage to the gate of N-MOSFET N_(C2) so that a current I_(L) constantly flows to and through the N-MOSFET N_(C2).

FIG. 10 is an input-output waveform diagram showing operations of the interface circuit shown in FIG. 9. As illustrated in FIG. 10, when an input signal V_(IN) is at a high level, a low level signal is output from an inverter 51, and the P-MOSFET P₃ is ON, whereas the N-MOSFET N₃ is OFF. Accordingly, as shown with arrow B in FIG. 9, the output current I_(H) flows to the terminating resistor R_(L) via the P-MOSFET P_(C2) and the P-MOSFET₃, thereafter a high level signal (V_(OH)) is output to the output terminal D_(O). When the input signal V_(IN) is at a low level, a high level signal is output from the inverter 51, and the P-MOSFET P₃ is OFF, whereas the N-MOSFET N₃ is ON. Accordingly, the output current I_(L), as shown with an arrow C in FIG. 9, flows to the terminating resistor R_(L) via the N-MOSFET N_(C2) and the N-MOSFET N₃, thereafter a low level signal (V_(OL)) is output to the output terminal D_(O).

The sense amp 31 included in the bias circuit 3 is operable in such a way that the output voltage V_(RP) of the P-MOSFET P_(X2) and the control voltage V_(P) coincide with each other. Thus, the current I_(RP) flowing to the fixed resistor R_(P2) is always and constantly controlled without being effected by any variation in the source voltage V_(DD) or any difference occurring in transistors in the manufacturing processes.

The P-MOSFET P_(X2) of the bias circuit 3 and the P-MOSFET P_(C2) of the drive circuit 5 are operable in their saturation range, and their dimensions are designed so as their constants to be set at a predetermined ratio. In such a structure, the P-MOSFET P_(X2) and the P-MOSFET P_(C2) are operable under a Miller effect, thus a current value of the current I_(RP) flowing to the P-MOSFET P_(X2) and a current value of the current I_(H) flowing to the P-MOSFET P_(C2) are in proportion to each other.

The sense amp 41 of the bias circuit 4 is operable in such a way that the output voltage V_(RN) of the N-MOSFET N_(X2) and the current voltage V_(N) coincide with each other. Thus, the current I_(RN) flowing to the fixed resistor R_(N2) is always constantly controlled without being effected by any variation in the source voltage or any difference occurring in transistors in the manufacturing processes.

The N-MOSFET N_(X2) of the bias circuit 4 and the P-MOSFET N_(C2) of the drive circuit 5 are operable in their saturation range, and the dimensions of the respective transistors are designed so that their constants are set at a predetermined ratio. Thus, the N-MOSFET N_(X2) and the N-MOSFET N_(C2) are operable under a Miller effect, a current value of the current I_(RP) flowing to the N-MOSFET N_(X2) and a current value of the current I_(L) flowing to the N-MOSFETP N_(C2) are in proportion to each other.

As illustrated in FIG. 9, the bias circuit 3 includes the sense amp 31 which constantly controls the current I_(RP) to flow to the fixed resistor R_(P2), any variation in a current I_(RP), as a result of a variation in a source voltage V_(DD) or any difference (deviation) occurring in transistors in the manufacturing processes, can be reduced. Thus, any variation in the output current I_(H) of the drive circuit 5, whose value is proportional to the value of the current I_(RP), can also be reduced.

The bias circuit 4 includes the sense amp 41 which constantly controls the current I_(RN) to flow to the fixed resistor R_(N2), reducing any variation in the current I_(RN) as a result of a variation in the source voltage V_(DD) or any difference (deviation) occurring in transistors during the manufacturing processes. This achieves a reduction in a variation in the output current I_(L) of the drive circuit 5 whose value is proportional to the current I_(RN).

The interface circuit according to the second embodiment does not have the structure of the second example, shown in FIG. 2, in which voltages of the output terminals D_(O) and X_(DO) included in the drive circuit are fed back to the sense amp. Thus, the interface circuit according to the second embodiment is operably always in a stable manner. Thus, unlike the second example, no noise occurs in the interface circuit according to the second embodiment.

As explained above, according to the prevent invention, a value of a control voltage for constantly controlling an output current to flow and a value of an output voltage of a transistor included in the bias circuit are controlled to be at a predetermined ratio, by the differential amplifier. By doing this, the current flowing to and through the fixed resistor and the current flowing to the transistors in the bias current are constantly controlled to flow, reducing any variation in the circuit which variation may occur as a result any variation in the source voltage or any difference occurring in transistors in the manufacturing processes.

According to the structure of the present invention, a reduction in a variation in the current flowing to the transistor which is operable under a Miller effect together with the transistors in the bias circuit can be achieved, and a reduction in a variation in the current flowing to the terminating resistor can be achieved.

Because the voltage of the output terminal is not fed back to the differential amplifier, such an amplifier can be operable always in a stable manner, preventing any noise from occurring in the amplifier.

Various embodiments and changes may be made thereonto without departing from the broad spirit and scope of the invention. The above-described embodiments are intended to illustrate the present invention, not to limit the scope of the present invention. The scope of the present invention is shown by the attached claims rather than the embodiments. Various modifications made within the meaning of an equivalent of the claims of the invasion and within the claims are to be regarded to be in the scope of the present invention.

This application is based on Japanese Patent Application No. H11-149787 filed on May 28, 1999, and including specification, claims, drawings and summary. The disclosure of the above Japanese Patent Application is incorporated herein by reference in its entirety. 

What is claimed is:
 1. An interface circuit which outputs an output signal in accordance with an input signal to at least one end of a load, said interface circuit comprising: said load; a driving circuit having a first transistor which supplies a first constant current to a switching circuit which supplies said first constant current to said load; and a bias circuit comprising: a fixed resistor; a second transistor which is connected with said fixed resistor and which is operable with said first transistor under a Miller effect; and a control circuit which controls a voltage applied to control terminals of said first and second transistor and an output voltage of said second transistor to be at a predetermined voltage ratio.
 2. The interface circuit according to claim 1, wherein: each of said first and second transistor includes a current path and a control terminal; said current path of said first transistor is connected to said switching circuit; said current path of said second transistor is connected to said fixed resistor; and wherein said control circuit comprises: a differential amplifier whose inverting input terminal receives a predetermined voltage, and whose non-inverting input terminal receives a voltage of a connection node between said second transistor and said fixed resistor, and an output terminal which applies an output voltage at said control terminals of said first and second transistor.
 3. The interface circuit according to claim 1, wherein, said switching circuit switches a flow direction of a current flowing through said load from one end to the other end and from the other end to the one end.
 4. The interface circuit according to claim 1, further comprising: a second driving circuit having a third transistor which supplies a second constant current to said load and a second switching circuit which supplies said second constant current to said load; a second bias circuit having a second fixed resistor, a fourth resistor which is connected with said second fixed resistor and which is operable with said third transistor under a Miller effect; and a second control circuit which controls a voltage applied to said control terminals of said third and fourth transistors and an output voltage of said fourth transistor to be at a predetermined voltage ratio.
 5. The interface circuit according to claim 4, wherein: each of said third and fourth transistors includes a current path and a control terminal; said current path of said third transistor is connected to said second switching circuit; said current path of said fourth transistor is connected to said second fixed resistor; said interface circuit further comprises: a second differential amplifier whose inverting input terminal receives a second predetermined voltage, and whose non-inverting input terminal receives a voltage of a connection node between said fourth transistor and said second fixed resistor; and an output terminal which applies an output voltage at said control terminals of said third and fourth transistors.
 6. The interface circuit according to claim 1, wherein said first and second transistors respectively comprise P-channel MOSFET's or N-channel MOSFET's.
 7. The interface circuit according to claim 4, wherein each of said first and second transistors respectively comprise P-channel MOSFET's or N-channel MOSFET's; and said third and fourth transistors respectively comprise N-channel MOSFET's or P-channel MOSFET's.
 8. An operating method of an interface circuit which outputs an output signal in accordance with an input signal to at least one end of a load, said operating method comprising: controlling and applying a common voltage to gates of a first transistor and a second transistor whose current path is connected to a resistor so that a voltage at a connection point of said resistor and said second transistor is a predetermined value, thereby to make said fist and second transistors operate under a Miller effect; and supplying, in response to said input signal, a current which flows through said first transistor and a switching circuit to said load, thereby outputting said output signal between both ends of said load or at one end of said load.
 9. The operating method of an interface circuit according to claim 8, further comprising: controlling and applying a second common voltage to gates of a third transistor and a fourth transistor whose current path is connected to a second resistor so that a voltage at a connection point of said second resistor and said fourth transistor is a second predetermined value, thereby to make said third and fourth transistors operate under a Miller effect; and supplying, in response to said input signal, a current which switchingly flows through said first transistor and said third transistor to said load. 